Alternatively, you might look at adapting the epic spellcasting system for spells from 0th to 9th level for use when someone wants to try casting an unmemorized spell. It would take some work to do it right, but it might work out pretty well.

Of course, these are just some ideas, there are naturally lots of other ways you might consider as well.

But mostly penalty on to hit. If it does not roll to hit but requires a saving throw then the saving throw has a modifier for non memorized. If it does not require a to hit or a saving throw then I might need to work out a spell failure system for non memorized spells, such as buffs.

Or maybe just a base spell failure system for all non memorized spell regardless on which roll it requires.

That said, which roll are you penalizing, or does it depend on the spell? Lots of spells don’t require a roll to hit on the part of the caster. Would you apply this penalty as a bonus to saving throws instead for these spells?

Or are you talking more about damage rolls? If so, at low levels, the penalty might be significant, but at high levels it would be very negligible. Also, not all spells cause damage. Particularly buff spells. For those, you just cast them and they happen. No one would bother memorizing buffs under a system like this.

If that’s the game you’re looking for, then by all means, give it a try and see how it works. I’m just not sure how thought out this is, given the wide array of spells and the different ways in which those spells are used.

That, or I simply don’t understand what you were getting at.

Either way, good luck with your game.